Aussie Testarossa rebuild

I made a start on shimming my diff tonight.

First step is to set the pinion backlash. To do this, start off with a shim under the ground down and removable bearing cup on the pinion side of the gearbox. I started with my smallest one first. 2.4mm

What you are looking for is movement at the outside teeth of the crownwheel of between 0.12mm and 0.17mm (5 thou is close to 0.127mm) My aim was for 0.0052 thou or 0.132mm. To do this you set up a dial gauge indicator on the very tip of the crownwheel tooth and as you rock the crownwheel you can feel (and hear) the slop and this movement is recorded on the dial gauge.

I worked up through my shims and settled on a 2.78 shim which gave me exactly 0.132mm, just where I wanted to be. More by good luck than good management.

Next step it to set the bearing preload. You do this by altering the large outer shims under the RHS diff carrier assembly.

I have very few of these to work with, 0.33, 1.33, 1.6, and 1.78.

1.78, too tight. 1.78 + 0.33, still too tight. Bugger!

1.33 + 1.60 too loose by far. With this combination in place I set up the dial gauge to check the end float or how much the diff moved in and out with this combination. Hmmm 20 thou or 0.508mm.

So I need to remove 1/2 a mm from my stack and then I should at least start to get some preload.

I'll continue on tomorrow working with shim material and feeler gauges to try to come up with the right number. I'm guessing at 2.04mm shim at this early stage. Stand by>>>>>>>
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I'm going to struggle Mike. I have nowhere near enough variation in available shims to allow me to do it by the more traditional methods.

Tried calling you earlier, will try again tomorrow.

 

Well the diff is in and shimmed up.

I played with it a bit to get a feel but once I was on top of things it actually took about four hours to do it.

Factory preload and backlash and a beautiful blue mesh pattern to the crown wheel and pinion teeth.

A variety of tools required to do this job, but my mechanic loaned me a fantastic yet simple tool produced by SnapOn called a torque meter which was invaluable.

Here's some pictures of the three places that I took various measurements to double and triple check the bearing preload and the final assembled diff in place.

I actually didn't have sufficient shims available to complete the large external shim measurement, so as you can see in the pictures I used four sets of feeler gauges that could be screwed apart to extract the various blades. In the picture below you can see four 16 thou (0.4064mm) have been used to calculate the final shim measurement.
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Testing bearing preload at the actual crown wheel bolts. This area has a mechanical advantage very similar to that discussed in the factory manual.
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One thing very important for you blokes to note.

When I started setting bearing preload, there was an area where the diff was binding when I was rotating it. It was nice and easy to rotate and then for a short length it was stupidly tight, like goind from 3 Nm or torque to nearer 15 - 17 Nm required to rotate the diff.

I couldn't quite get this, so after many tries I blued, rotated and then removed the crown wheel to check pattern on the pinion head and the crown wheel teeth, thinking that either this was causing the issues, or that my bearings were miss aligned for some reason.

Once refitted without the crown wheel installed, the diff preloaded fine and rotated perfectly smooth.

Issues with crownwheel I suspected. Not good!!

So I grabbed a sheet of 10mm industrial float glass that I keep around and I taped three sheets of abrasive paper to the glass. 400#, 800# and 1200# finishes.

Starting with the 400 dry i did a circular / figure 8 roatation with the rear of the crown wheel on the paper. Hmm, high and low spots. Like WTF is that about.

Took a while and then I moved up to 800 and 1200 grits in we paper, but eventually the rear of the crownwheel was perfectly flat and gleaming.

Sure enough, there had been warping of the crown wheel. Probably caused by factory tightening but there was certainly a slight cupping of the rear face and a few bolt holes were high spots.

Once refitted and torqued to factory spec of 70 Nm, the diff was installed and spun 100% perfectly. All binding dissapeared and the entire job became easier from that point on.

NEVER trust your old crown wheel. Check it for flat before fitting.

 

Hi Phil.

I reckon I may have had an advantage that most installers wouldn't have. With the engine not only out of the car and also out of the frame, to make it even easier, it's upside down on a stand with 100% total access to both sides of the diff. Can't get any easier than that.

I could change the small shim under the bearing cup and try another in less than two minutes.

Gotta love it when a plan comes together!

 

Yes, it was certainly interesting. To think that factory torque specs may have actually twisted or distorted the crown wheel to the extent that three of the bolts points had actually pulled. When I started to sand the back of the crown wheel, instantly it was obvious that three or four of the bolt holes were higher.

Didn't take too much to get it flat, but I'm certainly glad that I did it. The diff ran so much more freely once that was done.

 

Yes. the crown wheel and pinion are engraved with the same numbers, a matching set.

And yes, the originally crown wheel was very slightly warped and at least three, if not four of the threaded bolt holes had been pulled upwards causing the crown wheel not to mesh perfectly with the pinion.

 

Away for a few days with work from tomorrow but did a few small jobs today.

Here's the dial gauge set up to check backlash.
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And I fitted some of the newly coated pipes back today.
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I seem to be getting somewhere, but this box has over 1200 pieces that are still required to be fitted back into the jigsaw puzzle.

I just hope that Alzheimers doesn't set in!